The City of San Rafael lies in what is considered "Earthquake Country". On the basis of research conducted since the 1989 Loma Prieta earthquake, the U.S. Geological Survey (USGS) and other scientists conclude that there is a 70% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2030. Major quakes may occur in any part of this rapidly growing region. This emphasizes the urgency for all communities in the Bay region to continue preparing for earthquakes.
Earthquakes are caused when faults beneath the ground surface rupture and suddenly slip. When the rupture extends to the surface of the ground, it is considered a Strike-slip Fault. This causes the ground on one side to move past the ground on the other side of the fault. Most of the earthquake faults in the San Francisco Bay Area are Strike-slip Faults. The Hayward Fault segments and the Peninsula segment of the San Andreas Fault are felt to have a probability of one in four of producing a magnitude 7 or greater earthquake.
Geological Materials and Shaking Amplification
A critical factor affecting damage to a specific site is the geological material under the site. Much of the Bay Area and a narrow easterly band of the City of San Rafael are built on Bay mud and fill. These areas will experience the worst damage because of the deep, loose solid, which amplifies and prolongs the shaking.
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